"It is considered extremely unlikely that this operation could have affected the mammals in any way," the navy said in a statement. The ship was mapping the seabed about 14 miles off the southwest coast of England.
But Marine Connection, a whale and dolphin protection charity, insisted the underwater noise could have disoriented the animals and pushed them into dangerous waters.
"Something has definitely spooked these animals," said Liz Sandeman, its director of operations.
The common dolphins — which usually shy away from the coast in favor of deeper waters — were found beached in and around a creek off the Percuil River, near Falmouth, on Monday. Some were rescued, but 26 dolphins suffered painful, protracted deaths.
Campaigners say post-mortem examinations showed that the animals appeared healthy and their stomachs were empty, indicating they were scared into heading up the river.
"They weren't coming up the river system to fish, which leads us to suspect that they were frightened up the river system," said Sarah Dolman, a science officer with the Whale and Dolphin Conservation Society.
Dolman said there were no documented cases linking a mass stranding to side-scan sonar — which are directed downward toward the ocean floor, unlike mid-frequency active sonar, which are directed outward into the water.
However, "we're learning more all the time and it's still a possibility," she said.
Rodney Coates, an expert on underwater acoustics, said the sonar could well have hurt the dolphins, who use similar frequencies to locate their prey. Coates said that if the dolphins' hearing was affected, they were doomed.
"Sound is to the dolphin what sight is to you," Coates said. "A deaf dolphin will be a dead dolphin, it's only a question of time."
Sonar can also affect whales, and court-imposed restrictions bar the U.S. Navy from using active sonar within 2,000 yards of the mammals.
Commanders argue the restrictions are hampering sonar training and say they could eventually degrade its military readiness. The matter could be headed to the U.S. Supreme Court.
A British military spokesman, speaking on condition of anonymity in line with military policy, said he could not offer immediate comment on any restrictions placed on the use of naval sonar in Britain.
"We as the Royal Navy take our responsibilities toward the environment seriously, and the use of sonar is controlled accordingly," he said.
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